Heavy Armor Kit vs. Fortifier on a Shell

Back in the action after more than a year away from War Robots. Damn, how things change. Anyway, I now have a Shell and am trying to decide between Heavy Armor Kits x 3, or Fortifier x 3.

I have been running Fortifier thus far, and leveling them up regularly, but saw an Adrian video where he had 3 Heavy Armor Kits on his Shell, I think it amounted to 431k damage durability, and now I am wondering which would be the best fit?

When trying to understand how both work, these are the questions I have...

1) Fortifier - physical shield bonus is x% and regeneration bonus is x%.  Does that regeneration bonus actually regenerate a physical shield, or does it only work on energy shields?

2) Heavy Armor Kit - Does the Heavy Armor Kit reinforce the Shell's physical shield, or is it just pure durability applied to the robot itself and not the shield?

3) So if I read this right, with Fortifier the Shell will have a stronger physical shield, but with Heavy Armor Kit the Shell itself sans the physical shield will be stronger right?

Of the two options, which choice allows the Shell to hang in the fight longer?

Thanks!